Mantle overturn and oxygenation of the atmosphere

The presence of abundant oxygen in Earth’s atmosphere defies Le Chatelier’s Principle – it should react rapidly with the rest of the environment through oxidation.  That it does not is sufficient evidence for an alien observer to conclude that our planet is dominated by photosynthetic life at its surface and the burial of carbohydrate by geological processes.  So, Le Chatelier is not defied on the long term, because the CO2 + H2O = carbohydrate + oxygen equilibrium does not reach a balance because of continual removal of organic material from the right-hand side!  That Mars has no atmospheric oxygen bears witness to its lifelessness in that respect, as concluded decades back by James Lovelock.

Before 2.5 Ga ago, in the Archaean, atmospheric oxygen was a trace gas.  Preservation of detrital grains of sulphides and uranium oxides in Archaean clastic sequences, that would have broken down in an oxidizing environment, is the main evidence for that.  The other side of the coin is that oxygen-producing photosynthesizers – the cyanobacteria – were abundant throughout the Archaean, leaving their trace as common stromatolitic carbonates and signs of the crucial enzyme rubisco in kerogens and the carbon-isotope record.

If cyanobacteria generated oxygen, then why did it not build up in the atmosphere throughout the Archaean, instead of from about 2.2 Ga ago?  The most likely explanation is that Archaean magmatism released vast amounts of Fe-II or ferrous iron to sea water, which then reacted with available oxygen to form the ferric oxide of banded iron formations (BIFs), with the biproduct of hydrogen gas that further drove Archaean environmental chemistry into a reducing condition.  Seawater circulating through Archaean ocean crust would also have enriched basalts in ferric iron by the same oxidizing reaction.  Such a chemical model still leaves unexplained the shift to an oxygenated atmosphere after the Archaean.

Norman Sleep of Stanford University, reviews an article by Kump et al. in  Geochemistry, Geophysics, Geosystems (2001) that deals with this dilemma (Sleep, N.H.  2001.  Oxygenating the atmosphere.  Nature, v. 410, p. 317-319).  Kump and his co-workers suggest that, rather than relating to a change in palaeoecology, the shift arose from subduction of dense ferric oxide-rich lithosphere to settle at the core-mantle boundary.  By the end of the Archaean oxidized material filled the lower mantle.  Heating reduced its density so that it became buoyant.  If that deep oxidized layer rose to displace more primitive, reducing mantle, later magmatism would have released less Fe-II, thereby allowing biologically generated oxygen to build up.  The converse effect would have been to bring down levels of reducing atmospheric gases, such as hydrogen, methane and carbon monoxide, to trace levels.

Except to its primitive producer – cyanobacteria – oxygen would have been anathema to the dominant anaerobic Bacteria and Archaea that constituted Archaean life.  An end-Archaean mantle overturn, implicated by the tectonic pandemonium from 2.7 Ga, could well have triggered accelerated extinction and evolution that encouraged the rise of the eukaryote cell that requires oxygen for its basic metabolism.  Nonetheless, such an upheaval would have been directly connected with earlier living processes.  That is something which will delight followers of the Gaia hypothesis.

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